Semiconductor chips (semiconductor elements) used for electronic control have been increasingly reduced in size, and the amount of heat generation from the chips grows steadily. Accordingly, it is important to improve heat dissipation in a semiconductor circuit board (including a module) mounted with semiconductor chips. The reason for this is that if a semiconductor chip exceeds the intrinsic temperature thereof even at only one point of the chip, the resistance of the chip changes to a negative-side temperature coefficient. This change causes a thermal runaway in which a power current flows intensively, thus instantaneously destroying the chip. That is, there is a demand for making heat dissipation designs in consideration of margins consistent with the electrical power loss of chips.
Thermal resistance (Rth) can be expressed by the formula Rth=L/(k×A). In the formula, Rth is thermal resistance, L is a heat transfer path, k is thermal conductivity, and A is a heat dissipation area. According to this formula, the thermal resistance (Rth) becomes lower with a decrease in the heat transfer path (L) and an increase in the thermal conductivity (k) and the heat dissipation area (A). Note that in general, the heat transfer path (L) corresponds to the thickness of a circuit board.
A commonly-known semiconductor device using a semiconductor chip involves contact among dissimilar materials, where the heat transfer path of the semiconductor device is represented as chip→solder→electrode circuit material→insulating substrate→back-side metal plate→solder→heat-dissipating member (heat sink). These heat mediums, except the heat-dissipating member, belong to an insulating circuit board. That is, the performance improvement of the semiconductor device cannot be achieved unless the insulating circuit board which accounts for most of the heat transfer path is superior in heat dissipation performance.
An Si chip which is the mainstream of current semiconductor chips has encountered its limit of response speed. Accordingly, the development of chips using SiC and GaN as next-generation semiconductor elements is moving ahead at a fast pace as a national project, with the aim of further downsizing equipment and improving the performance thereof. An SiC chip, in particular, is said to be usable at a temperature of up to 600° C., whereas the operable temperature of the mainstream of Si chips is 125 to 150° C. Thus, in addition to fast response speed, high operating temperature is another characterizing feature of the SiC chip.
If a conventional solder material is used to bond the chip and an electrode circuit material, however, the operating temperature decreases to the melting point of the solder material or lower. It is therefore not considered possible to take advantage of the chip's feature of being high in operating temperature. Under the current circumstances where a high-melting point solder material is being developed, any solder materials having a melting point of 600° C. or higher and sufficient reliability have not yet developed. A pamphlet of International Publication No. WO2007/105361 (Patent Document 1), for example, proposes using an Ag—Cu brazing material in place of high-melting point solder materials. It has been confirmed that use of such a high-melting point brazing material having a junction temperature of 600° C. or higher as described in Patent Document 1 enhances the reliability of junction (bonding) between the electrode circuit material and the semiconductor element to a certain degree.
On the other hand, attempts are being made to not only enhance the reliability of the junction but also further improve heat dissipation. For example, studies are being made of a method for directly bonding the semiconductor chip to the electrode circuit material without the interposition of any jointing materials, such as a brazing material, and a method for dissipating heat not only in a latitudinal direction but also in a lateral direction by thickening the electrode circuit material. In addition, whereas a single-sided cooling method in which a semiconductor chip is joined (bonded) to a heat sink through an insulating circuit board is the mainstream in a conventional semiconductor circuit board module, a double-sided cooling method or the like in which the semiconductor chip is cooled from both sides thereof has been put in use.